A. Field of the Invention
The present invention relates to determining the location of a moving radio transmitter using fixed passive radio receivers.
B. Description of the Prior Art
Accurate methods for determining the position of a wireless transmitter have long been sought. Wireless positioning systems in the past were primarily used for military, public safety and marine science research. Recently, commercial services have proposed useful applications for positioning technologies in such areas as accident reporting, mobile yellow pages, carrier zone billing and fleet management.
An exemplary use of wireless position is the FCC""s mandate that by Oct. 1, 2001, all wireless carriers provide the longitudinal and latitude coordinates of any wireless E911 caller. The goal of this mandate is to provide the location of the caller to within a root means square (RMS) distance of 125 meters.
As discussed in xe2x80x9cA Selective Model to Suppress NLOS Signals in Angle-of-Arrival (AOA) Location Estimationxe2x80x9d, by Xiong (IEEE, Sep. 9, 1998), which is hereby expressly incorporated herein by reference, one proposed method for location estimation is called Angle of Arrival, also known as directional finding. The position of a transmitter is estimated based on the angles of incoming signals detected at fixed base stations. In order to determine the latitude and longitude of a transmitter via triangulation, it is necessary to use at least two base stations. In order to determine a position in three dimensions (latitude, longitude and height), at least a third base station is required.
In xe2x80x9cTwo Decades of Array Signal Processing Researchxe2x80x9d by Krim et al. (IEEE Signal Processing, July 1996, Vol. 13, No. 4), which is hereby expressly incorporated herein by reference, the authors discuss various methods for determining the propagation direction from a transmitter to an array receiver. A first method disclosed involved xe2x80x9csteeringxe2x80x9d the array in one direction at a time and measuring the received power. The locations of maximum power reception provide the angle of arrival estimate.
FIG. 1 shows graphically how AOA is performed using two base stations. Base stations 100 and 110 each receive a signal from radio transmitter 120. Base station 100 determines the angle xcex81 to transmitter 120 and base station 110 determines the angle xcex82 to transmitter 120. Each angle is used to create a line of bearing (LOB) in the direction of the user. The position at which the LOBs intersect is an estimation of the position of transmitter 120.
A number of error sources control the accuracy of AOA location estimation. These error sources include: a bias in antenna placement; background noise in the angular measurement due to multipath signals; and lack of a direct path between the transmitter and the receiver, called non line-of-sight (NLOS) measurements. Non-line-of-sight signals may include errors from reflection and diffraction, such that the measured direction is different from the location of the signal source.
Antenna placement bias can be mitigated with the periodical calibration and maintenance of the antenna system. A number of high-resolution algorithms have been proposed to counter the effects of background noise in the angular measurement. The inventor of this application has suggested viable method has been proposed to counter the effects of NLOS in AOA measurements.
The objects of the invention are to mitigate the effects of NLOS on angle of arrival measurements for location estimation.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes a method of estimating the position of a transmitter using a direction estimation from each of a plurality of receivers including steps of calculating a first position estimation using each of the direction estimations, calculating an error in the direction estimation for each receiver, selecting those receivers whose measurement error is below a threshold, and calculating a second position estimation using the direction estimations of the selected receivers.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description, serve to explain the principles of the invention.